70 



MAMMALIAN DESCENT. 



[LKCT. III. 



the upper region of the mouth, and which is formed by 

 a^ special' ingrowth of bone from the maxillaries and 

 palatines, is large and imperfect where the four plates 



FIG. 9. Skull of an embryo Pig, 3 inches long, lower view, magnified 3 diameters. 

 al.n, alse nasi (snout cartilage); e.n, external nostrils ; px., premaxillary ; mx, 

 maxillary ; d.g, dental groove (for tooth-pulps) ; pa, palatine ; pg, pterygoid ; v, 

 vomer ; e.pg, external pterygoid plate ; 6s, basi-f phenoid ; g.f, glenoid fossa (hinge 

 cartilage for lower jaw) ; ty, tympanic bone ; ob, os-bullse (additional tympanic 

 bone) ; bo, basi-occipital ; p.oc, paroccipital process ; oc.c, occipital condyle 

 (hinge for first vertebra or atlas) ; eo, exoccipital ; so, superoccipital ; f.m, 

 foramen magnum (great opening) ; IX. X. XII., holes for the exit of the 9th, 10th, 

 and 12th cranial nerves. 



should meet. It is less imperfect in the embryo than 

 afterwards, but the bone-cells are very thinly scattered 



